The general problem:
"Browser functionalities such as bookmarks and the "back" button allow users to request pages from web applications in unpredictable ways. When incorrectly handled, an unespected request from the user can execute an unintended action, produce a cryptic and confusing error message, or even expose details about the application that can be exploited for malicious purposes" (Hallé, Ettema, Bunch and Butan, 2010*)
The particular problem:
"The developer therefore becomes resposible of writing customized mechanisms for these (...) features, a tedious and cumbersome task" (Ibidem).
The particular defect in Fat Free:
"In an MVC application, the handling of navigation constraints should clearly be the responsibility of the controller. However, MVC-based applications do not fully exploit this feature" (Ibidem).
This seems to be the case of Fat Free framework: there are no plugins or mechanisms to respond to this problem, very actual nowaday, for example in SaS.
In conclusion it seems to be no solution offered to such a problem in the current version of Fat Free.
Source (*) http://www.cs.ucsb.edu/~bultan/publications/ase10.pdf